Retina.js
JavaScript and LESS helpers for rendering high-resolution image variants
Respond
A fast & lightweight polyfill for min/max-width CSS3 Media Queries (for IE 6-8, and more)
Jo
Jo is a thin (~12K) candy shell for PhoneGap apps. It's an HTML5 mobile app framework which provides UI widgets, a flexible event model, a wrapper for sound, and a light data layer. Apps are skinnable with CSS3, and work everywhere PhoneGap does... and then some, including Adobe Air and OSX (desktop and dashboard).
Jquery UI
The official jQuery user interface library.
jPlayer
jPlayer : HTML5 Audio & Video for jQuery
JS Signals
Custom Event/Messaging system for JavaScript inspired by AS3-Signals
jScrollPane
Pretty, customisable, cross browser replacement scrollbars
JSON 3
A modern JSON implementation compatible with nearly all JavaScript platforms.
JSON Editor Online
A web-based tool to view, edit and format JSON
Kerning.js
Take control of your web typography.
jStorage
jStorage is a simple key/value database to store data on browser side. As a bonus it is also a local pubsub platform to sync data between tabs/windows
Knockout
Knockout makes it easier to create rich, responsive UIs with JavaScript
LABjs
Loading And Blocking JavaScript: On-demand parallel loader for JavaScript with execution order dependencies
Lazyload
Tiny, dependency-free async JavaScript and CSS loader.
Leaflet
JavaScript library for mobile-friendly interactive maps
Less.js
Leaner CSS
Lodash
An alternative to Underscore.js, delivering consistency, customization, performance, and extra features.